The instant invention is founded in the discovery that nickel oxide (NiO) has a unique ability to retard or totally suppress TiO.sub.2 -induced nucleation of the low temperature crystal phases in alkali metal aluminosilicate glasses, viz., beta-quartz solid solution (beta-eucryptite) in lithium aluminosilicate glasses and carnegieite (Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) in low silica, sodium or sodium-potassium aluminosilicate glasses. Hence, the heat treatment of such glass compositions, also containing TiO.sub.2, commonly results in the conversion of the glass to a glass-ceramic article. However, the inclusion of NiO inhibits the normal growth of silicate crystals during heat treating of the glass and, instead, causes a color change from brown to green to be effected. Colors ranging from light yellow-green to dark emerald to blue-green can be obtained, depending upon glass composition and heat treatment. The glassy products are transparent and, particularly in the case of Li.sub.2 O-containing compositions, can be chemically strengthened to very high values, utilizing low temperature exposures to baths of molten salts.
The heat-induced color shift from brown to green indicates a change in coordination of the Ni.sup.+2 ion from tetrahedral to octahedral. Whereas the inclusion of both NiO and TiO.sub.2 in the glass composition is essential for the development of the green coloration, the NiO inhibits TiO.sub.2 from performing its customary function as a nucleating agent for beta-quartz solid solution (s.s.) or carnegieite. It must be recognized, however, that the glassy, low crystalline, green state is metastable with respect to temperature. Sufficiently elevated temperatures will transform the Li.sub.2 O-containing, green glassy material to transparent, reddish-brown beta-quartz s.s. glass-ceramic articles and, at still higher temperatures, opaque beta-spodumene s.s. glass-ceramics will be produced. In like manner, the transparent Na.sub.2 O-containing glassy materials will first be converted to carnegieite glass-ceramic articles and, at still higher tempratures, nepheline glass-ceramics will be produced.
The ability of NiO to inhibit nucleation of TiO.sub.2 in this manner appears to be unique. Thus, the phenomenon was not observed with other transition metal oxides such as MnO, FeO, CoO, or CuO. Glasses having compositions in the alkali metal aluminosilicate field containing any one or more of those oxides with TiO.sub.2 were converted in the normal manner to beta-quartz s.s. or carnegieite glass-ceramics. Furthermore, no unusually colored intermediate glassy state was observed.
Although the reaction mechanism through which NiO suppresses nucleation is not completely understood, both the suppression of nucleation and the development of green color are associated with the growth of a small amount, i.e., less than about 5% by volume, of very fine-grained crystallites of nickel spinel (NiO.Al.sub.2 O.sub.3) during the prescribed heat treatment of the glass. This compound is well-developed in nickel-containing beta-spodumene s.s. and nepheline glass-ceramic bodies and imparts a bluish-green coloration thereto. X-ray diffraction analyses of the inventive green glasses have identified two very weak, diffuse, diffraction peaks, the intensities of which vary with the NiO content of the composition. These peaks, centered at d-spacings of about 2.43A and 2.01A, correlate to the two strongest lines of NiO.Al.sub.2 O.sub.3. It is known in the crystallographic literature that NiO.Al.sub.2 O.sub.3 is an "inverse" spinel wherein the Ni.sup.+2 ions occupy octahedrally coordinated positions, consistent with the green color.
The essentially complete freedom from haze exhibited by the green glass of the invention and the diffuse nature of the diffraction peaks are evidence that the size of the spinel crystallites is less than the wavelength of visible light, i.e., less than a few thousand angstroms in diameter.
It is postulated that the formation of the nickel spinel must in some way upset the nucleation process. Three possible mechanisms underlying the action of NiO have been hypothesized. First, the presence of NiO may prevent the development of Al.sub.2 Ti.sub.2 O.sub.7, believed to be the source of nuclei in the alkali metal aluminosilicate glass system. Second, a solid solution of NiO, Al.sub.2 O.sub.3, and TiO.sub.2 having the structure of spinel may be formed. Third, TiO.sub.2 may nucleate the nickel spinel preferentially to beta-quartz s.s or carnegieite. Any of those events might well result in the suppression of the conventional TiO.sub.2 nucleation of silicate crystal phases.